Methods for forming lead zirconate titanate nanoparticles

ABSTRACT

Methods for forming lead zirconate titanate (PZT) nanoparticles are provided. The PZT nanoparticles are formed from a precursor solution, comprising a source of lead, a source of titanium, a source of zirconium, and a mineraliser, that undergoes a hydrothermal process. The size and morphology of the PZT nanoparticles are controlled, in part, by the heating schedule used during the hydrothermal process.

CROSS REFERENCE TO RELATED APPLICATIONS

This nonprovisional patent application is related to contemporaneouslyfiled U.S. nonprovisional patent application Ser. No. ______ titled“METHOD AND SYSTEM OF FABRICATING PZT NANOPARTICLE INK BASEDPIEZOELECTRIC SENSOR”, having Attorney Docket Number 11-0195-US-NP,filed on Aug. 17, 2011, and this nonprovisional patent application isalso related to contemporaneously filed U.S. nonprovisional patentapplication Ser. No. ______, titled “METHOD AND SYSTEM FOR DISTRIBUTEDNETWORK OF NANOPARTICLE INK BASED PIEZOELECTRIC SENSORS FOR STRUCTURALHEALTH MONITORING”, having Attorney Docket Number 11-0839-US-NP, filedon Aug. 17, 2011. The contents of both of these contemporaneously filedU.S. nonprovisional patent applications are hereby incorporated byreference in their entireties.

BACKGROUND

Lead zirconate titanate (PZT) is a piezoelectric ceramic material thatis widely used for its high piezoelectric coefficients. PZT has aperovskite structure with a chemical formula as ABX₃, where A is lead, Bis mixture of zirconium and titanium, and X is oxygen. PZT appears as asolid with phases of lead zirconate oxide (PbZrO₃) and lead titanateoxide (PbTiO₃).

Formation of PZT films typically requires sintering at a hightemperature (e.g., 650° C.), which makes application of PZT difficult onnon-planar (e.g., “three dimensional”) substrates because the resultingfilms are brittle (because they are sintered) and difficult to pattern.

What is desired, therefore, is a relatively low temperature method forforming PZT, such that PZT can be deposited conformally on non-planarsubstrates.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

In one aspect, a method is provided for forming a plurality of PZTnanoparticles (also referred to herein as “nanocrystals”). In oneembodiment, the method includes the steps of:

(a) providing an aqueous precursor solution comprising a mineralisersolution, a source of titanium, a source of zirconium, and a source oflead; and

(b) heating the precursor solution to produce PZT nanoparticles, whereinheating the precursor solution comprises a first heating schedule thatincludes at least the sequential steps of:

-   -   (i) heating the precursor solution at a first rate to a first        temperature, wherein said first rate is greater than about 1°        C./min, and wherein said first temperature is between about        120° C. and about 350° C.;    -   (ii) holding for a first hold time at the first temperature,        wherein said first hold time is between about 5 and about 300        minutes; and    -   (iii) cooling at a second rate to provide a nanoparticle PZT        solution comprising a suspended plurality of perovskite PZT        nanoparticles having a smallest dimension of between about 20 nm        and about 1000 nm, wherein said second rate is greater than        about 1° C./min.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 graphically illustrates a heating schedule as used in the methodsfor forming PZT nanoparticles in accordance with embodiments providedherein;

FIG. 2 illustrates a portion of a heating schedule, including a slowramp rate, as used in the methods for forming lead zirconate titanate(PZT) nanoparticles in accordance with embodiments provided herein;

FIG. 3 illustrates a portion of a heating schedule, including a fastramp rate, as used in the methods for forming PZT nanoparticles inaccordance with embodiments provided herein;

FIG. 4 graphically illustrates x-ray diffraction (XRD) data obtainedfrom PZT nanoparticles formed in accordance with embodiments providedherein;

FIGS. 5A-5D are scanning electron micrographs of PZT nanoparticlesformed in accordance with embodiments provided herein;

FIGS. 6A-6D are scanning electron micrographs of PZT nanoparticlesformed in accordance with embodiments provided herein;

FIGS. 7A-7F are scanning electron micrographs of PZT nanoparticlesformed in accordance with embodiments provided herein;

FIGS. 8A-8E are scanning electron micrographs of PZT nanoparticlesformed in accordance with embodiments provided herein;

FIGS. 9A-9E are scanning electron micrographs of PZT nanoparticlesformed in accordance with embodiments provided herein; and

FIGS. 10A-10E are scanning electron micrographs of PZT nanoparticlesformed using excess lead in accordance with embodiments provided herein.

DETAILED DESCRIPTION

Methods for forming lead zirconate titanate (PZT) nanoparticles areprovided. The PZT nanoparticles are formed from a precursorsolution—comprising a source of lead, a source of titanium, a source ofzirconium, and a mineraliser—that undergoes a hydrothermal processaccording to the following reaction (“the hydrothermal process”):

Pb²⁺ +xTiO₂+(1−x)ZrO₂+2OH⁻⇄PbTi_(x)Zr_(1-x)O₃+H₂O

The formed PZT nanoparticles are useful, for example, as piezoelectricmaterials.

Previous syntheses of PZT particles demonstrate the tremendous syntheticchallenges in size control, size distribution, morphology, andagglomeration of formed PZT particles. According to the providedmethods, the characteristics of the formed PZT nanoparticles arecontrolled to a greater extent than previously obtainable. Of particularinterest is providing a plurality of PZT nanoparticles with the smallestparticle size while maintaining the highest possible purity of theperovskite PZT structure.

The fabrication of the smallest possible PZT nanoparticles is of use,for example, if the PZT nanoparticles are to be incorporated into asolution for solution deposition (e.g., using a spray nozzle) to producea large area coating. Spray nozzles are as small as hundreds ofnanometers in diameter, meaning that if PZT nanoparticles in solutionare to be sprayed through such a nozzle, the PZT particles must be atleast half the size of the nozzle.

In the provided methods, the properties of the formed PZT nanoparticlesare dictated at least by the mineraliser concentration, processing time,heating rate, and cooling rate.

In one aspect, a method is provided for forming a plurality of PZTnanoparticles (also referred to herein as “nanocrystals”). In oneembodiment, the method includes the steps of:

(a) providing an aqueous precursor solution comprising a mineralisersolution, a source of titanium, a source of zirconium, and a source oflead; and

(b) heating the precursor solution to produce PZT nanoparticles, whereinheating the precursor solution comprises a first heating schedule thatincludes at least the sequential steps of:

-   -   (i) heating the precursor solution at a first rate to a first        temperature, wherein said first rate is greater than about 1°        C./min, and wherein said first temperature is between about        120° C. and about 350° C.;    -   (ii) holding for a first hold time at the first temperature,        wherein said first hold time is between about 5 and about 300        minutes; and    -   (iii) cooling at a second rate to provide a nanoparticle PZT        solution comprising a suspended plurality of perovskite PZT        nanoparticles having a smallest dimension of between about 20 nm        and about 1000 nm, wherein said second rate is greater than        about 1° C./min.

Precursor Solution

The precursor solution is defined by reactants that are processed toform PZT nanoparticles. Specifically, the precursor solution includes atleast a source of titanium, a source of zirconium, a source of lead, anda mineraliser. The precursor solution optionally includes additionalsolvents or stabilizers, as will be discussed in more detail below.

The components of the precursor solution may all be combinedsimultaneously in a single reaction vessel, or may be combined stepwise,depending on the character of the components of the precursor solutionand a potential need to minimize interaction between the components ofthe precursor prior to hydrothermal reaction to produce PZTnanoparticles. For example, as set forth below in Example 1, the sourceof titanium and the source of zinc may be combined to form a precursorgel, which is then combined with a source of lead in aqueous form andthe mineraliser to provide the precursor solution. Such an approachallows for maximum control over the relative molar amounts of each ofthe reactants (i.e., the sources of titanium, zirconium, and lead).

The sources of lead, titanium, and zirconium are present in theprecursor solution in molar amounts sufficient to obtain PZTnanoparticles having the formula Pb_(x)Zi_(y)Ti_(z)O₃, wherein x isbetween 0.8 and 2, wherein y is between 0.4 and 0.6, and wherein y plusz equals 1. For example, a common formula for perovskite PZTnanoparticles is Pb(Zr_(0.52)Ti_(0.48))O₃. However, it will beappreciated by those of skill in the art that the relative amounts oflead, zirconium, and titanium can be modified within the provided rangesto produce the desired characteristics of PZT nanoparticles.

The source of titanium in the precursor solution can be anytitanium-containing compound that allows for the formation of PZTparticles according to the method provided herein. In one embodiment,the source of titanium is Ti[OCH(CH₃)₂]₄. Additional sources of titaniuminclude TiO2, TiO2*nH2O, Ti(OC4H9), Ti(NO3)2, TiCl3, and TiCl4.

The source of zirconium in the precursor solution can be anyzirconium-containing compound that allows for the formation of PZTparticles according to the method provided herein. In one embodiment,the source of zirconium is Zr[O(CH₂)₂CH₃]₄. Additional sources ofzirconium include Zr(NO3)4*5H2O, ZrOCl2*8H2O, ZrO2*nH2O, and ZrO2.

The source of lead in the precursor solution can be any lead-containingcompound that allows for the formation of PZT particles according to themethod provided herein. In one embodiment, the source of lead isPb(CH₃COOH)₂. Additional sources of lead include Pb(NO3)2, Pb(OH)2, PbO,Pb2O3, and PbO2.

In certain embodiments, excess lead is added to the precursor solution.As used herein, the term “excess lead” refers to a ratio amount greaterthan one for the source of lead. Excess lead is a means for exertingfurther control over the characteristics of the PZT nanoparticles.

Typically, the excess lead is achieved in the precursor solution byadding an excess amount of the same source of lead as described above.For example, if the source of lead is lead acetate trihydrate, anyamount of lead acetate trihydrate added to the precursor solution thatresults in the ratio of the lead acetate trihydrate to be greater thanone compared to the source of zirconium and the source of titanium willbe considered an excess amount of lead. In certain embodiments, theexcess amount of lead comes from a second, different, source of lead.

Excess lead does not alter the chemical composition of the PZTnanoparticles, but instead modifies the hydrothermal reaction conditionsto produce several desirable effects on the formed PZT nanoparticles.While the excess lead does not alter the fundamental crystal structureof the PZT nanoparticles, it improves morphology, reduces amorphousbyproducts, and reduces the degree of agglomeration between particles.The specific benefits of excess lead are set forth below in Example 2.

One less desirable side effect of excess lead is that it also leads tothe formation of a lead oxide compound that is an impurity. However, thelead oxide impurity can be removed by washing the formed PZTnanoparticles with an appropriate solvent (e.g., diluted acetic acid).

The mineraliser in the precursor solution facilitates the formation ofPZT during the hydrothermal process. The mineraliser acts as a source ofhydroxide ions to facilitate the hydrothermal synthesis of PZT.Representative mineralisers include KOH, NaOH, LiOH, NH₄OH, andcombinations thereof. The concentration of the mineraliser in theprecursor soultion is from about 0.2 M to about 15 M. The optimalmineraliser concentration depends on the conditions of the hydrothermalprocess, as is known to those of skill in the art.

The concentration of the mineraliser affects the size of PZTnanoparticles produced. For example, as disclosed below in Example 1,similar PZT nanoparticles formed using 5 M and 10 M KOH mineraliser havesimilar morphology, but 5 M mineraliser results in smaller nanoparticlesthan those formed with 10 M mineraliser, if all other processingconditions are the same.

In certain embodiments, a stabilizer is added to the precursor toprevent gelation and/or precipitation of certain components of theprecursor prior to the hydrothermal process. That is, stabilizers may berequired to maintain all of the necessary components of the precursor insolution prior to the hydrothermal process. For example, in oneembodiment, acetylacetone (“AcAc”) is added to the source of titanium(e.g., titanium isopropoxide) to prevent gelation and precipitationprior to reaction to form PZT. Another stabilizer useful in the methodsis propoxide.

The precursor solution is typically aqueous, although it will beappreciated that any other solvent capable of solvating the componentsof the precursor solution and facilitating the formation of PZTnanoparticles can also be used. For example, the precursor solution canbe aqueous, a mixture of water and an organic solvent, or a weak organicacid. Exemplary non-water components of the solution includeethylenediamine, CH₂Cl₂, ammonium salt, and acetic acid.

In an exemplary embodiment, described further in Example 1, theprecursor solution comprises KOH as the mineraliser solution, titaniumisopropoxide as the source of titanium, zirconium n-propoxide as thesource of zirconium, lead acetate trihydrate as the source of lead,acetylacetone as a stabilizer, and water. The lead acetate trihydrate,zirconium n-propoxide, and titanium isopropoxide are present in theprecursor in a weight ratio of from about 1 to about 2 parts leadacetate trihydrate, from about 0.5 to about 1 parts zirconiumn-propoxide, and from about 0.8 to about 1.6 parts titaniumisopropoxide. The KOH mineraliser solution is from about 0.2 to about 15M.

Heating Schedule

PZT nanoparticles are formed through hydrothermal processing of theprecursor solution. The hydrothermal process includes a heating schedulecomprising a heating ramp to a first temperature, a hold at the firsttemperature, and a cooling ramp to room temperature, as is graphicallyillustrated in FIG. 1. The temperatures referred to herein arespecifically the temperature of the precursor solution itself.

The heating schedule is performed under pressure greater than 1 atm.Accordingly, the precursor solution is contained within an apparatusconfigured to both heat and pressurize. In certain embodiments, thepressure applied during the heating schedule is from about 1 atm toabout 20 atm. In an exemplary embodiment, the precursor solution iscontained within an autoclave and autogenous pressure builds in theautoclave over the course of the heating schedule. Alternatively, aconstant pressure can be provided by a pump or other apparatus known tothose of skill in the art. The heat and pressure required for the methodcan be provided by any means. An exemplary system includes a digestionbomb (configured to contain high pressures) within a furnace (configuredto provide heat).

In one embodiment, heating the precursor solution to produce PZTnanoparticles includes at least the sequential steps of:

(i) heating the precursor solution at a first rate to a firsttemperature, wherein said first rate is between about 1° C./min andabout 30° C./min, and wherein said first temperature is between about120° C. and about 350° C.;

(ii) holding for a first hold time at the first temperature, whereinsaid first hold time is between about 5 and about 300 minutes; and

(iii) cooling at a second rate to provide a nanoparticle PZT solutioncomprising a suspended plurality of perovskite PZT nanoparticles havinga smallest dimension of between about 20 nm and about 1000 nm, whereinsaid second rate is between about 1° C./min and about 30° C./min.

The heating ramp rate (“first rate”) is used to raise the temperature ofthe precursor solution from about room temperature (T_(RT)) to themaximum hydrothermal processing temperature (T_(max)). In oneembodiment, the first rate is between about 1° C./min and about 30°C./min. In another embodiment, the first rate is greater than about 10°C./min.

The processing temperature (“first temperature”; T_(max)) is betweenabout 120° C. and about 350° C. In certain embodiments, the firsttemperature is 200° C. or less. While the heating schedule is primarilydescribed herein as including a single first temperature, to which thesolution is heated, it will be appreciated that the present methodcontemplates variations in the first temperature that may arise from thehydrothermal reaction or inaccuracies in the heating equipment.Furthermore, the heating step of the heating schedule may includesecond, third, or further temperatures to which the heated precursorsolution is subjected. The second, third, or further temperatures may behigher or lower than the first temperature, as required to produce thedesired PZT nanoparticles.

The first rate is particularly important to control the size of the PZTnanoparticles produced. In this regard, referring to FIGS. 2 and 3, asthe temperature of the precursor solution heats from T_(RT) to T_(max),there is an intermediate temperature, T_(nuc), at which PZT crystalsbegin to nucleate. This “Nucleation Zone” is illustrated in FIGS. 2 and3. Optimal PZT crystal growth occurs at T_(max), and any crystalsnucleated at a temperature lower than T_(max) will likely grow larger,form larger aggregates, and will agglomerate more than PZT crystalsnucleated at T_(max).

A slow ramp rate, as illustrated in FIG. 2 results in a longer amount oftime that the precursor solution spends between T_(nuc) and T_(max).Accordingly, a slow ramp rate results in more PZT crystal nucleation attemperatures below T_(max), resulting in inconsistent PZT crystal sizeand crystal structure. As used herein, the term “slow ramp rate” refersto a ramp rate of below 1° C./min.

Conversely, a relatively fast ramp rate, as illustrated in FIG. 3,results in homogeneous PZT crystal nucleation by passing the precursorsolution quickly through the temperature range between T_(nuc) andT_(max). As used herein, the term “fast ramp rate” refers to a ramp rateof 10° C./min or greater. In certain embodiments, the high ramp rate isa ramp rate of 20° C./min or greater.

As a result of the nucleation dynamics described above, the higher theramp rate, the smaller the PZT particles generated.

While the heating ramp rate affects the size and number of PZT crystals,it does not affect the crystal structure. Similarly, the cooling ratedoes not affect the crystal structure.

The “hold” step of the heating schedule allows the PZT crystals time toform and grow. The hold step is between about 5 and about 300 minutes atthe first temperature. Typically, a longer hold time results in largercrystals. If the hold step is too short, the PZT phase will not form.

After the hold step, the heating schedule proceeds to a “cooling” step.The cooling rate reduces the temperature from the maximum processingtemperature to room temperature at a “second rate.” In one embodiment,the cooling rate is greater than 1° C./min. In another embodiment, thecooling rate is from about 1° C./min to about 30° C./min.

With regard to forming quality PZT nanoparticles, the “initial coolingstage” is an important aspect of the method. As used herein, the term“initial cooling stage” refers to the period of cooling between thestart of the cooling step and when the precursor solution reaches 100°C., as illustrated in FIG. 1. In one embodiment, the cooling rate in theinitial cooling stage is greater than 1° C./min. In another embodiment,the cooling rate in the initial cooling stage is greater than about 10°C./min.

While the cooling rate illustrated in FIG. 1 is linear, it will beappreciated that the cooling rate need not be linear, and is typicallyexponential.

The cooling rate impacts several aspects of the PZT nanoparticles. Thecooling rate can be controlled by modifying the temperature surroundingthe precursor solution. For example, if the precursor solution is in adigestion bomb within a furnace for heating, the furnace can be turnedoff to produce a slow cooling rate, or the digestion bomb can be removedfrom the furnace and placed in a cool environment (e.g., an oilquenching bath) to provide a high cooling rate.

The cooling rate partially determines the morphology and size of theformed PZT nanoparticles. As described in further detail in Example 1, arelatively fast cooling rate, for example, a cooling rate of greaterthan 20° C. per minute, results in PZT nanoparticles in the range of 100nm to 500 nm and a distinct cubic shape, as illustrated in FIG. 8C.

Additionally, a relatively fast cooling rate results in PZTnanoparticles that are physically bonded, as opposed to chemicallybonded. Physically bonded PZT nanoparticles are preferable to those thatare chemically bonded because separation of physically bondednanoparticles is accomplished more readily than the separation ofchemically bonded nanoparticles (e.g., by mechanical agitation).

Finally, a faster cooling rate minimizes the presence of PbTiO₃ phase inthe final product. This is desirable because PbTiO₃ not only is animpurity that must be removed to obtain pure PZT nanoparticles, butforming PbTiO₃ also reduces the yield of the PZT-formation reaction byconsuming lead and titanium in a form other than PZT.

In certain embodiments, the second rate is sufficiently large that PZTparticles are formed that are non-perovskite forms of PZT. In thisregard, in certain embodiments, the method further comprises a step oftreating the nanoparticle PZT solution to eliminate the non-perovskiteforms of PZT. Such a treatment may include chemically-assisteddissolution, wet etching, acid washing, base washing, and combinationsthereof. Any method that selectively eliminates (e.g., dissolves) thenon-perovskite PZT can be used. For example, a dilute acetic acid washcan be used to eliminate the PbTiO₃ non-perovskite component of the PZThydrothermal synthesis.

Alternatively, instead of eliminating the non-perovskite PZT particles,in certain embodiments, the method further includes a step of separatingthe perovskite PZT nanoparticles from the non-perovskite forms of PZT inthe nanoparticle PZT solution. In this additional step, the endsuspension is washed with DI water, diluted acid, or ethanol to removethe non-perovskite forms.

In certain embodiments, the second rate is sufficiently large that thenanoparticle PZT solution becomes supersaturated. In this regard,nucleation and crystal growth proceeds when the solution issupersaturated and then stops when the concentration reaches anequilibrium. For all temperatures, there is a equilibrium concentrationthat halts crystal growth. Therefore, when the second rate is slow, thesolution can be supersaturated multiple times and the crystal has morechances to grow larger. Conversely, for a fast second rate, the solutiondoes not have time to reach equilibrium, which promotes furthernucleation to occur along with crystal growth. The nucleation rate ishigh when the concentration is high so both nucleation and growth arerapid. Because of this effect, the secondary nucleation and growth willnot form stable crystals or will create amorphous materials, both ofwhich are undesirable. Preferably, nucleation only occurs during at theend of the heating ramp and during the hold time.

As will be explained in further detail in Example 1, the route toforming the smallest and highest quality PZT nanoparticles is achievedusing the shortest possible processing time for the hydrothermalprocessing, which includes using the highest heating ramp rate, thefastest cooling ramp rate, and a mineraliser concentration thatfacilitates quality PZT formation. The ideal mineraliser concentrationaffects the processing time. For example, in an exemplary system, if 5Mmineraliser is used, the processing time can be as short as 1 hour butfor 2M mineraliser, the required processing time is 3 hours.Additionally, if the mineraliser concentration is not sufficient, no PZTwill form, no matter how long the processing time. For example, if themineraliser concentration is lower than 0.4M, no PZT will be formedregardless of the processing time.

After the cooling step, a PZT nanoparticle solution is obtained. The PZTnanoparticle solution contains a plurality of PZT nanoparticlessuspended in water. The PZT nanoparticle solution can be filtered orotherwise manipulated to isolate the PZT nanoparticles. Depending on theefficiency of the hydrothermal process, the solution may also containPbTiO₃, PbZrO3, PbO, TiO2, ZrO2, and KOH. Washing the solution withacetic acid is one method for removing PbO.

The PZT nanoparticles can be used for any application for whichtraditionally-formed PZT is used, as known to those of skill in the art.For example, the PZT nanoparticles can be used as a piezoelectricmaterial when integrated with appropriate electrodes, as known to thoseof skill in the art.

Furthermore, the formed PZT nanoparticles may also enable applicationsfor which traditional PZT cannot be used. For example, the PZTnanoparticles can be added as a guest to a host matrix to form acomposite that can be applied conformally to a surface as apiezoelectric material. In such a “guest-host” application, the PZTnanoparticles can be incorporated, for example, into a silica precursorsolution that is then spray-coated onto a surface to provide aPZT-silica composite film having piezoelectric properties. Because sucha film can be spray coated, composite can be deposited conformally onnon-planar surface.

The following examples are provided for the purpose of illustrating, notlimiting, the methods described herein.

EXAMPLES Example 1 Synthesis of PZT Nanoparticles

As a first step, Ti(OCH(CH₃)₂)₄ (tetra-iso-propyltitanate, “TIPT”) wasmixed with acetylacetone (“AcAc”) in a molar ratio of 2 AcAc to 1 TIPT(e.g., 2.860 mL of TIPT and 1.960 mL of AcAc) to stabilize the TIPT. Themixture was continuously stirred under room temperature for 4 hours, andzirconium acetate, Zr[O(CH₂)₂CH₃]₄ (4.660 mL) was added after stirring.Next, the mixture of Ti and Zr sources was added dropwise into a 1 Maqueous potassium hydroxide (KOH) solution. White precipitate was formedduring this process. Centrifugation was used to separate the whiteprecipitate from the residual KOH solution. The precipitate was washedwith DI water until it was pH neutral. The white precipitate (in theform of gel) was then mixed with lead acetate trihydrate, Pb(C₂H₃O₂)₂powder (1.4210 g). Next, the mixture was added into a second aqueous KOHsolution (serving as a mineraliser), whose concentration was varied tocontrol the properties of the resulting PZT nanoparticles.

After all chemicals were mixed, the precursor solution was sealed in anautoclave and subjected to hydrothermal processing in an oven at 200° C.and 15.549 bar.

During the hydrothermal process, various combinations of ramping rate,processing time and cooling time were used to obtain a suspension withPZT nanoparticles.

Finally, the suspension was centrifuged, washed until the pH wasneutral, and then the PZT particles were oven dried for evaluation, suchas by scanning electron microscopy (SEM) and x-ray diffraction (XRD).

The experiments were repeated with some parameters always being fixed,while others varied from trial to trial. The fixed parameters include(a) molar ratio of each chemical (except the mineraliser) (molar ratioPb:Ti:Zr=1:0.48:0.52) and (b) the amount of the PZT precursor injectedin the autoclave and the furnace during the hydrothermal growth (10 mLof precursor injected into a 20 mL autoclave).

The following parameters were varied during the trials. First, themineraliser concentration used was 0.4 M, 1 M, 5 M, and 10 M. Second,the heating ramp rate was 3° C./min, 10° C./min, or 20° C./min. Third,the hydrothermal processing time was 1 hour, 3 hours, 5 hours, 8 hours,and or 15 hours. Finally, three cooling rates are employed: slow,medium, and fast. For the slow cooling rate, the autoclave is cooleddown in the furnace with the furnace door closed; the slow cooling rateis about 1.6° C./min. For the medium cooling rate, the autoclave wasfirst kept in the closed furnace for some time and the furnace door waslater opened to expedite the cooling; the medium cooling rate is fromabout 1.6° C./min to 6.7° C./min. For the fast cooling rate, the furnacedoor was opened right after the hydrothermal process is completed; thefast cooling rate is above 10° C./min.

Table 1 summarizes the controlled factors in all trials.

TABLE 1 Conditions for Trials A-F Sample Process time Ramp rateMineraliser Cooling Trial # (hr) (° C./min) Concentration (M) Rate A 1 33 0.4 Slow 2 5 3 8 4 15 B 1 3 3 1 Slow 2 8 C 1 5 10 5 Slow 2 5 10 3 10 54 10 10 5 1 20 5 Slow 6 1 10 7 5 5 8 5 10 9 10 5 10 10 10 D 1 1 20 5Slow 2 Fast 3 4 5 E 1~5 1 20 5 Medium  6~10 Fast F 1~8 1 20 5 Fast

The PZT powders were evaluated with X-ray diffraction (XRD) and scanningelectron microscopy (SEM). XRD can not only identify existence of PZT,but also provide information about crystallinity and size of smallestrepeatable crystal cell of PZT. SEM reveals morphology and size of thePZT particles. The data and relevant FIGURES refer to samples labeledwith the following convention:

Trial (a letter)-processing time (hour)-ramping rate (°C./min)-mineraliser concentration (M)-cooling rate (S/M/F).

FIG. 4 shows XRD patterns from various trials. The figure consists ofvarious processing times, ramping rates, mineraliser concentrations, andcooling rates. PZT peaks have high intensity in each of the XRD patternsof FIG. 4, which indicates the presence of crystalline PZT.

In FIG. 4, impurities are also present in certain samples. For example,the slowly cooled samples include the PbTiO3 phase in addition to PZT.For example, FIG. 4 includes XRD patterns from Trial C, which consistsof short processing time, higher ramping, higher mineraliserconcentration, and slow cooling rate. Samples with 5M mineraliserconcentration all show PbTiO3 phase, regardless of the processing timeand ramping rate.

In contrast, Trials D and E consist of short processing time, highramping rate, medium mineraliser concentration (5 M), and variouscooling rates. These two trials were designed as repeating samples toconfirm the results. FIG. 4 includes XRD patterns from trial E, withmedium and fast cooling rates. Both patterns imply acceptablecrystallinity of PZT. The results from trial E also indicate that thedegree of peak shift is independent of the cooling rate. Finally, theseresults indicate that faster cooling minimizes the presence of thePbTiO₃ phase.

SEM images of Trials A, C, D, E, and F are discussed below. Trials withsimilar conditions are placed together for comparison.

FIGS. 5A-5D show SEM images of Trial A. With “slow” ramping rate and lowmineraliser concentration, all particles are larger than 1 μm. Ingeneral, the degree of aggregation and size of the crystal increase withthe processing time. For the sample of 15-hour processing time (FIG.5D), a larger cluster of small crystals is shown in the SEM images.

FIGS. 6A-6D show SEM images from Trial C, with a “medium” ramping rateof 10° C./min. All SEM images have the same magnification of 30,000. Ingeneral, all samples with a 5M mineraliser concentration have particlesize in the range of 200 nm to 600 nm. The processing time does notaffect the size of the crystal significantly. When the mineraliserconcentration is increased to 10M, longer processing time results inlarger crystals. When the processing time is less than 5 hours, themineraliser concentration does not affect the crystal size.

FIGS. 7A-7F show SEM images from Trial C, with a “fast” ramping rate of20° C./min. Again, samples with 5M mineraliser concentration showconsistent crystal size regardless of the processing time. Similarly,increase of mineraliser concentration promotes larger particle size.

According to Trial C, the highest ramping rate (20° C./min) and themedium mineraliser concentration (5 M) produce the smallest particlesize. Therefore, we use these parameters along with a short processingtime (1 hour) as a reference configuration for subsequent Trials D, E,and F, in which effects of cooling rates are studied.

FIGS. 8A-8E show SEM images from Trials D, E, and F. The effect ofcooling rate is explored in FIGS. 8A-8E. A “slow” cooling rate of about1.6° C./min was used to generate the samples of FIGS. 8A and 8D; a“medium” cooling rate of about 1.6° C./min to 6.7° C./min was used togenerate the samples of FIGS. 8B and 8E; and a “fast” cooling rate ofabove 10° C./min was used to generate the sample in FIG. 8C. The slowand medium cooling rates produces larger crystal in the range of 300 nmto 800 nm. Moreover, some slow and medium samples show crystals that arechemically bonded increasing overall particle size. In contrast, thefast cooling rate (FIG. 8C) leads to the best morphology. The crystalshape is uniform and the crystal structure can be seen clearly. The fastcooling rate results in smaller crystals in the range of 100 nm to 500nm. Moreover, any crystals that are bonded to other crystals arephysically bonded, instead of chemically bonded, as was seen in othersamples in this example. This distinction is important becausephysically bound crystals are relatively easily separated (e.g., bygrinding or sonication), whereas chemically separated crystals are moredifficult to separate (e.g., by chemical or electrochemical etching).

Overall, a combination of short processing time (1 hour), highestramping rate (20° C./min), medium mineraliser concentration (5 M), andfast cooling rate provides the smallest and best-formed PZTnanoparticles.

PZT nanoparticles were fabricated through use of hydrothermal process bycareful controlling four process parameters. Ramping and cooling ratesare two important parameters for controlling size and morphology.Mineraliser concentration, if properly selected, cannot only stabilizeparticle size, but also minimize effects of processing time on theparticle size. Processing time should be reduced to ensure that theparticle size is within the acceptable range. Overall, the bestparameter configuration for the samples tested in this example, consistsof short processing time (1 hour), highest ramping rate (20° C./min),medium mineraliser concentration (5M) and fast cooling rate. Under theseconditions, PZT nanoparticles with a size of less than 600 nm can beproduced with superior morphology and crystallinity.

Example 2 Synthesis of PZT Nanoparticles Using Excess Lead

The addition of excess lead in the precursor solution was investigatedas a means of controlling the size, morphology, and other features ofPZT crystals.

In this example, the reaction conditions were essentially the same asdescribed in Example 1. The primary difference is that greater than 1weight equivalent of the source of lead, Pb(C₂H₃O₂)₂, was used. Theamount of excess lead was varied in order to test the effect of excesslead. In the results provided herein, the amount of excess lead islisted as a percentage, with 0% being no excess lead (i.e., 1 weightequivalent of the source of lead), and 100% being two weight equivalentsof the source of lead. For example, if no excess lead is 5 g ofPb(C₂H₃O₂)₂, an 80% excess of lead would result from 9 g of Pb(C₂H₃O₂)₂.

Both 1 M and 2 M aqueous KOH mineraliser concentrations were used.

The hydrothermal conditions were 3 hours at 200° C., with 20° C./minramping rate, and the fastest cooling rate of above 10° C./min.

After the PZT was formed, the powder was washed with diluted acetic acid(10% vol.) to remove the lead oxide generated by the excess lead.

It was determined that when excess lead is used, the resulting (pre-acidwash) powder is pink or yellow suspension, as opposed to white for amore pure PZT suspension. The color is attributed to a lead oxide. Thesuspension turn white after washing with dilute acetic acid to removethe lead oxide. While the lead oxide is an unwanted byproduct of theexcess lead technique, the relatively easy step of an acid wash removesthe lead oxide.

It was determined that excess lead in the precursor solution requires alonger hydrothermal processing time to form the PZT. For example, 5Mmineraliser, 1 hr sample failed to have PZT composition even with only10% wt excess lead.

Regarding mineraliser concentration, 1 M mineraliser concentrationresults in two distinct morphologies: amorphous and non-detectably smallcrystals.

Furthermore, the 1 M samples failed to form PZT when the excess lead isabove 10%. Accordingly, the results presented herein were obtained using2 M mineraliser.

When compared to no excess lead (i.e., 0% excess lead), the excess lead:reduces the amount of amorphous material in the PZT suspension; the PZTparticles are less agglomerated; the PZT particles are less chemicallybonded into aggregates; the PZT particles have improved morphology(e.g., more individual crystals, a sharper cubic structure, andwell-developed facets); the PZT particle size increases as the amount ofexcess lead increases; and the final suspension still contains both aPbZrO₃ phase and a PbTiO₃, in addition to PZT.

For the excess Pb trial, increasing the percentage of excess lead cansignificantly reduce amorphous phase. Also, all samples with excess leadshow less agglomeration, fewer large (i.e., greater than one micron)chemically bonded aggregates, better morphology. However, as percentageof excess lead increases, the particle size increases as well. Thesample without excess lead and 10 wt. % excess sample have particle sizeabout 200 nm. The 20 wt. % and 40 wt. % excess samples have particlesize about 400 nm. The 80 wt. % excess sample has particle size around600 nm. FIGS. 9A-9E (10,000×) and FIGS. 10A-10E (30,000×) are SEMmicrographs of PZT nanoparticles formed using various amounts of excesslead in the precursor solution.

While illustrative embodiments have been illustrated and described, itwill be appreciated that various changes can be made therein withoutdeparting from the spirit and scope of the invention.

1. A method for forming a plurality of piezoelectric perovskite leadzirconium titanate (PZT) nanoparticles, the method comprising the stepsof: (a) providing a precursor solution comprising a mineraliser,titanium isopropoxide, zirconium n-propoxide, lead acetate trihydrate,acetylacetone, and water, wherein the lead acetate trihydrate, zirconiumn-propoxide, and titanium isopropoxide are present in the precursor in aweight ratio of from about 1 to about 2 parts lead acetate trihydrate,from about 0.5 to about 1 parts zirconium n-propoxide, and from about0.8 to about 1.6 parts titanium isopropoxide; and (b) heating theprecursor solution to produce PZT nanoparticles, wherein heating theprecursor solution comprises a first heating schedule that includes atleast the sequential steps of: (i) heating the precursor solution at afirst rate to a first temperature, wherein said first rate is greaterthan about 1° C./min, and wherein said first temperature is betweenabout 120° C. and about 350° C.; (ii) holding for a first hold time atthe first temperature, wherein said first hold time is between about 5to about 300 minutes; and (iii) cooling at a second rate to provide ananoparticle PZT solution comprising a suspended plurality of perovskitePZT nanoparticles having a smallest dimension of between about 20 nm andabout 1000 nm, wherein said second rate is greater than about 1° C./min.2. The method of claim 1, wherein the first rate is sufficiently fastthat the PZT nanoparticles do not nucleate in the precursor solutionduring heating.
 3. The method of claim 1, wherein during the first holdtime the PZT nanoparticles nucleate to provide a nanoparticle solutioncomprising a suspended plurality of PZT nanoparticles.
 4. The method ofclaim 1, wherein the second rate is sufficiently fast that thenanoparticle PZT solution becomes supersaturated.
 5. The method of claim1, wherein the second rate is sufficiently fast that PZT particles areformed that are non-perovskite forms of PZT.
 6. The method of claim 5,further comprising a step of treating the nanoparticle PZT solution toeliminate the non-perovskite forms of PZT using a treatment selectedfrom the group consisting of chemically-assisted dissolution, wetetching, acid washing, base washing, and combinations thereof.
 7. Themethod of claim 6, further comprising a step of separating theperovskite PZT nanoparticles from the non-perovskite forms of PZT in thenanoparticle PZT solution.
 8. The method of claim 1, wherein themineraliser is selected from the group comprising KOH, NaOH, LiOH,NH₄OH, and combinations thereof.
 9. The method of claim 1, wherein themineraliser has a concentration of between about 0.2 M and about 15 M.10. The method of claim 1, wherein the first heating schedule isperformed in a pressure-controlled atmosphere having a pressure ofbetween about 1 atm and about 20 atm.
 11. The method of claim 1, whereinthe plurality of perovskite PZT nanoparticles have the formulaPb_(x)Zi_(y)Ti₃O₃, wherein x is between 0.8 and 2, wherein y is between0.4 and 0.6, and wherein y plus z equals
 1. 12. The method of claim 1,wherein the lead acetate trihydrate, zirconium n-propoxide, and titaniumisopropoxide are present in the precursor in a weight ratio of greaterthan 1 to about 2 parts lead acetate trihydrate, from about 0.5 to about1 parts zirconium n-propoxide, and from about 0.8 to about 1.6 partstitanium isopropoxide.
 13. The method of claim 1, wherein the first rateis greater than about 10° C./min.
 14. The method of claim 1, wherein thesecond rate is greater than about 10° C./min.